This invention relates to improvements in maintaining cargo containers under surveillance, and more particularly to theft preventing apparatus capable of responding to unauthorized movement of, or entry into, various objects, such as trucks, trailers cargo containers, and the like. The invention is especially adapted for use at a truck-trailer depot, railroad yard, or other transportation terminals so as to enable guards and other authorities to respond effectively to any unauthorized movement of the cargo either with or without the container or vehicle or to temperature changes within a cargo container.
In the transportation of goods by truck, by truck-trailer vehicles, or by railroad cars, the bodies or containers are often stored for periods of time in depots or terminals where they await further transfer or movement of the contents into or out of such bodies or containers. The high risk problem which results from such storage makes surveillance essential to the custody or control of the vehicles or containers or their contents.
Personal surveillance has proven too costly and/or ineffective for safeguarding valuable items, particularly cargo during transit or when located at a yard or dock. Locking devices, including padlocks, are not effective as they can be removed or rendered inapplicable by tools and technology in the hands of the criminal element, and the entire container or truck-trailer is frequently transported without authorization from the transportation terminal.
One of the features of the present invention resides in the selection of a carrier wave frequency which is subject to less signal attenuation caused by environmental conditions of freight terminals, yards, docks, and the like which are associated with cargo transportation industries. The commonly used frequencies at 88 or 300 MHz are subject to being attenuated or blocked by intervening metallic structures between the transmitter and receiving antenna, especially at the relatively low power levels which are dictated by the physical size and acceptable cost limitations imposed upon the transmitter units that are adapted to be attached to each cargo container that is placed under surveillance. An external antenna or radiating element, which can be rendered ineffective, will of course destroy the operativeness of the monitoring system, and it is therefore not practical to employ a large antenna.
Since the containers, which may be the trailer for a truck or box car, have widths and heights that are on the order of nine feet, it has been found that by employing a radio frequency carrier signal having a wavelength of approximately four times the dimension of the cargo container, it is possible to utilize a small element which can be contained within the transmitter package in conjunction with the metallic sidewall of the cargo container in a manner such as to immensely increase and enhance the radiation characteristics of the transmitted signal far beyond that which could be produced by the use of a conventional loop type antenna.
Attempts have been made heretofore to monitor and protect selected items with a variety of systems and components, but these have not been considered entirely satisfactory. In some cases, the system failed under the environment of field operating conditions, or because of inherent component inability to monitor specific motions or conditions against which protection is needed. Some systems have an inherently low level of security in protection, constitute violations of Government regulations during operation, or they provide insufficient control over operating personnel to be effective. Elaborate equipment with sophisticated systems involve prohibitive economics that thwart or limit extensive operations by potential users in the transportation industry.